The present invention relates to operational amplifier circuits. More particularly, the present invention relates to a circuit configured to enhance the slew rate of an operational amplifier.
In today""s world of high speed data transmission using electronic devices, there is an ever increasing need for improved operational amplifiers. In particular, significant efforts are being undertaken in the improvement of the response time and speed of operational amplifiers. Unfortunately, a variety of limitations are inherent in currently available operational amplifier topologies, and these limitations tend to limit the response time and speed capabilities of operational amplifiers.
Typically, operational amplifiers are configured to produce an output voltage in response to a set of input conditions. When those input conditions are modified, an operational amplifier typically responds by exhibiting a modified output voltage. A transition of an operational amplifier from an initial output state to a modified output state may be described in terms of two phases, namely a slewing phase and a decay phase. The response time of the operational amplifier is the time required for the operational amplifier to achieve a stable final output voltage in response to an instantaneous (e.g., step function) change in input conditions, i.e., to complete both the slewing phase and the decay phase.
The slewing phase of the output voltage transition begins upon the initiation of the input change and concludes when the output voltage approximates its final value. This phase is characterized by an elevated and substantially constant rate of change of output voltage with respect to time (i.e., an elevated slew rate). The decay phase begins upon the conclusion of the slewing phase and concludes when output voltage stabilizes within a tolerance range of its final value. The decay phase is typically characterized by decreasing rates of change of output voltage with respect to time. As the decay phase proceeds, the rate of change of output voltage with respect to time diminishes, and the output voltage settles.
The quickest possible slewing phase would be exhibited in an operational amplifier wherein the slewing characteristic approximates a step change, e.g., wherein the slewing time is infinitely small and the slew rate infinitely great, such as a vertical slew rate. Unfortunately, such slewing characteristics do not typically provide for a smooth transition from the slewing phase to the decay phase. Decay phases following very abrupt or steep slewing phases often exhibit instabilities, including extended oscillations about the final voltages, as well as significant overshooting of the final voltages. As a result, although a slewing phase may be accomplished relatively quickly, a decay phase following an overly fast slewing phase may be prolonged, causing an increase in response time. Accordingly, when attempting to improve response speed, increasing the slew rates, i.e. following a steep slew rate, without providing for a smooth transition to the decay phase may be counterproductive.
In a typical operational amplifier, the time required to accomplish a change in output voltage in response to a change in input voltage is directly related to the time required to change the voltage of the corresponding compensation capacitors of the operational amplifier. For a fixed current, that time is directly related to the capacitance of the compensation capacitors, which is also directly related to the stability of the amplifier. Accordingly, decreasing the capacitance of a device""s compensation capacitors while maintaining the rate at which current is supplied to the compensation capacitors will typically cause an increase in the rate at which the voltage of the compensation capacitors will change, resulting in an increase in slew rate and a decrease in the time of the slewing phase.
Unfortunately, however, such modifications typically cause decreases in device stability and often increases the time required to complete the decay phase. Also, operational amplifiers are frequently used to buffer the outputs of other devices. For example, an operational amplifier may be coupled to the output of a digital-to-analog converter (xe2x80x9cDACxe2x80x9d) so as to buffer the output of the DAC. In such cases, the particular DAC may specify certain desired or required output buffer characteristics in terms of, for example, acceptable capacitance and/or resistive loading. Such specifications may impose additional difficulties in achieving acceptable response time and stability characteristics and may affect the response time of the output device.
Accordingly, it would be desirable to increase the rate at which current is supplied to the compensation capacitors while maintaining their capacitance. Yet the prior art does not provide and practical and effective means for increasing the current supply rate. For example, in operational amplifiers comprising class A input stages, the current that may be available to charge the compensation capacitors may be fixed by the input stage. As a result, several techniques have been developed in an attempt to augment the current supplied by the input stages. All such prior attempts, however, have resulted in various adverse effects such as increased steady-state bias, offset voltage degradation, increased device complexity, and increased power dissipation.
For example, one such effort, aimed at increasing the rate at which current is supplied to the compensation capacitors, is disclosed in U.S. Pat. No. 4,783,637 to Cotreau (xe2x80x9cCotreauxe2x80x9d). The Cotreau patent describes a slew enhancement approach that employs a large signal slew enhancement stage connected in parallel to a small signal front-end stage of an operational amplifier. A parallel differential pair is used to monitor the input voltage, to detect a large signal condition, and to direct the current of the differential pair to the capacitor that most limits the response speed of the amplifier. Accordingly, when the inputs to the operational amplifier are changed, the slew enhancement stage is activated, and the slew rate of the device increases. Unfortunately, however, the Cotreau device requires an additional constant bias current for the large signal detector and requires added circuit complexity in order to steer current to the compensation capacitors.
Another attempt to improve the slew rate is disclosed in U.S. Pat. No. 4,701,720 to Monticelli (xe2x80x9cMonticellixe2x80x9d). The Monticelli device detects the output condition of an operational amplifier and feeds a corresponding signal back into a bias circuit in an attempt to improve the slew rate of the amplifier. The Monticelli device uses a capacitor to couple the output signal to the bias stage to affect the bias current. Unlike the Cotreau device, which detects and responds to changes in the input condition, the Monticelli device detects and responds to changes in the output condition, which necessarily lag the changes in the input condition. Accordingly, the Monticelli device often exhibits slower response characteristics than the Cotreau device.
Accordingly, it would be advantageous to have a circuit and method for increasing the slew rate of an operational amplifier without adversely affecting the response time of the amplifier or necessitating increased circuit complexity, such as a plurality of successive slewing phases, with each successive phase exhibiting a decreased slew rate.
The method and circuit according to the present invention address many of the shortcomings of the prior art. In accordance with various aspects of the present invention, an improved method and circuit are provided which can increase the slew rate of an operational amplifier without adversely affecting its response time. The method and circuit can also provide the ability to control the increase in current supplied to the compensation capacitors while also providing a smooth transition to the decay phase.
In accordance with an exemplary embodiment of the present invention, an operational amplifier is provided with a slew rate enhancement circuit comprising a large signal detector, a bias circuit having a bias override component, and a bias decay circuit providing a continuous (i.e., smooth) decay for the bias override component. When the input to the operational amplifier changes sufficiently to exceed a predetermined threshold, the large signal detector activates the bias override component of the bias circuit, which increases the current supplied to the compensation capacitors of the operational amplifier. In this way, the large signal detector and the bias override component increase the rate at which the compensation capacitors are charged, thereby increasing the slew rate.
Once the compensation capacitors substantially achieve their desired charge state, the amplifier feedback causes the input differential signal to decrease to a level below an activation threshold of the large signal detector. Rather than immediately transitioning to a lowered slew rate in accordance with the steady-state bias of the input stage, however, a decay of the bias is provided that allows an increased slew to persist for a defined period of time and then a smooth transition to steady-state bias levels in accordance with a stable amplifier. Thus, although the exemplary slew rate enhancement circuit accommodates increased slew rate, it does so without adversely affecting the decay phase and without necessitating the implementation of a plurality of successive slewing phases with decreasing slew rates, which would increase the complexity of the device.